1. Field of the Invention
This invention relates to a process for hydrogenating block copolymers. This invention particularly relates to a process for hydrogenating block copolymers using a heterogeneous catalyst.
2. Background of the Art
Unsaturated polymers are hydrogenated for a variety of reasons, frequently using a nickel catalyst. The presence of olefinic double bonds in the polymers makes them susceptible to oxygen attack and to deterioration by actinic radiation; saturation of olefinic double bonds greatly improves environmental stability and may improve color. For example, U.S. Pat. No. 3,130,237, suggests hydrogenating unsaturated compounds by the use of certain cobalt complexes. U.S. Pat. No. 3,205,218 teaches hydrogenation of olefinic compounds utilizing a complex of a nickel or cobalt compound with certain aluminum reactants. U.S. Pat. No. 3,113,986 is related and suggests hydrogenation in the presence of the reaction products of certain metal alkoxides with aluminum trialkyls. U.S. Pat. No. 3,595,942 teaches selective hydrogenation of block copolymers with reaction products of aluminum trialkyls and metal alkoxides or carboxylates. Finally, there is a great number of references on other hydrogenation catalysts.
A common problem shared by all types of hydrogenated polymers is the deleterious effect of catalyst residues remaining after hydrogenation. The quantity of metal residues to be removed may be as high as 5% wt. Nickel, for example, discolors the product and may cause polymer deterioration. It must therefore be removed almost completely in order to prepare polymers that are color stable.
One typical method employed in the art to remove catalyst residue is by mixing a solution or emulsion of polymer, sometimes referred to as a polymer cement, with an aqueous solution of an acid, usually a mineral acid. For instance, U.S. Pat. No. 4,396,761 describes a process of removing catalyst residue by treating the polymer cement with dilute sulfuric acid, separating the aqueous phase, and contacting the polymer-containing phase with live steam to drive off the polymer solvent and convert the polymer into polymer crumb. It also describes a process wherein an alpha-hydroxysulfonic acid is used instead of the dilute sulfuric acid, and wherein the polymer crumbs so produced are then subjected to a heat treatment that removes the alpha-hydroxysulfonic acid residues.
Use of heterogeneous catalysts for hydrogenation is also known. U.S. Pat. No. 3,415,759 describes the use of heterogeneous catalysts such as nickel on supports such as kieselguhr (diatomaceous earth) and Raney nickel. U.S. Pat. No. 5,028,665, U.S. Pat. No. 5,110,779, and U.S. Pat. No. 6,376,622 describe using heterogeneous catalyst that includes a group VIII metal on a porous support.
The use of acid washes such as those described above is necessitated by the use of homogenous catalysts and heterogeneous catalysts that are too small to filter. It would be desirable in the art of preparing block copolymers to prepare the block copolymers with heterogeneous catalysts that are large enough to filter and can be used to economically prepare the block copolymers.